Research Interests Research in our laboratory is devoted to understanding the mechanisms by which entire regions of the genome are rendered inaccessible to transcription and recombination. We employ genetic analysis coupled with biochemical fractionation and reconstitution experiments to explore the issues of genome accessibility. Current Research Silencing of genomic domains requires a complex series of interactions between silencers and repressor proteins. The silencers recruit repressor protein complexes composed of the Sir proteins that interact with histones in nucleosomes to form a chromatin domain that is inaccessible and inert to various cellular processes. We are presently working on 1) The biochemical and molecular characterization of the structure of the silent domain and its reconstitution in vitro 2) The mechanism by which the silent domain is restricted to a specific region of the genome and 3) The role of histone variants in cell cycle progression. We have been purifying Sir protein complexes and have purified complexes reconstituted from sub-units expressed in baculovirus infected insect cells. We have begun studies on the reconstitution of silenced chromatin using these complexes and histones in nucleosomes. These studies will provide an important index of our current understanding of transcriptional silencing depending on whether or not it is possible to reconstitute the silenced state, since mechanisms are rarely established by genetic means and usually require biochemical tests. In addition to these studies we are also interested in understanding the mechanism by which the silenced chromatin domains are restricted to specific regions along the DNA fiber. We have demonstrated that specific elements act as barriers to the continuous spread of the silenced chromatin. We performed a genome wide screen for proteins that can block the spread of silenced chromatin and analyzed the molecular mechanism by which specific native barriers function to block silencing and our results suggest two overlapping mechanisms function to restrict the spread of silencing, one of which involves a DNA binding element, while the second mechanism involves specific chromatin modifying activities. We have also begun determining the role of folding of chromatin in restricting the spread of silenced chromatin. We are currently interested in understanding the many roles of a histone H2A variant called Htz1p in the cell. Classical molecular genetic and biochemical experiments are in progress to determine the role of Htz1p in transcription, DNA replication and cell cycle progression and the inter-relationships between these processes. Together, these studies will allow a better understanding of the process of silencing and yield new insight into the mechanism by which genes are silenced.